The wreckage of the tour bus shortly after it overturned, killing seven people and injuring 25, in Phetchaburi early on Monday morning. (Photo by Chaiwat Satyaem)

Seven people were killed and 25 others injured when a double-decker tour bus bound for Bangkok from Chumphon overturned in Muang district early on Monday, police said.

Pol Lt Surakij Woranam, duty officer of Muang district police station, said the accident occurred about 3.15am on a curve near Ban Hua Don in tambon Hua Saphan.

Police and rescuers found four men and two women dead at the scene. One of the men was the driver, identified as L, 46. The other five were not yet identified.

Twenty-five passengers were injured. They were admitted to Phra Chomklao, Khao Yoi, Muang Phet-Thonburi and Phetcharat hospitals. One of the injured died at a hospital, taking the confirmed death toll to seven.

Phum Arunchote, 41, a passenger, said the bus departed Chumphon about 9.30pm. As the bus was nearing tambon Hua Saphan in Phetchaburi's Muang district he felt it was wobbling and going faster, prompting him to buckle up the seat belt.

At a curve near Ban Hua Don, the bus ran off the road, hit a power pole and overturned. He said most of the passengers had not fastened their safety belts and were thrown out of their seats by the violent crash.

The power post was broken by the impact, causing a brief power outage in the area.

Pol Lt Surakij said he initially believed the accident happened because the driver was asleep at the wheel. An investigation would establish the actual cause, he added.